Group III nitride semiconductor device and light-emitting device using the same

ABSTRACT

An object of the present invention is to provide a Group III nitride semiconductor device exhibiting improved crystallinity and a good performance. The inventive Group III nitride semiconductor device comprises a substrate, and a plurality of Group III nitride semiconductor layers provided on the substrate, wherein a first layer which is in contact with the substrate is composed of silicon-doped AlxGa1-xN (0≦x≦1). Also, the inventive Group III nitride semiconductor device comprises a substrate, and a plurality of Group III nitride semiconductor layers provided on the substrate, wherein a first layer which is in contact with the substrate is composed of AlxGa1-xN (0≦x≦1), and the difference in height between a protrusion and a depression which are present at the interface between the first layer and a second layer provided thereon is 10 nm or more and is equal to, or less than, 99% the thickness of the first layer.

CROSS REFERENCE TO RELATED APPLICATION

This application is an application filed under 35 U.S.C. §111(a)claiming benefit, pursuant to 35 U.S.C. §119(e) (1), of the filing dateof the Provisional Application No. 60/532,924 filed on Dec. 30, 2003,pursuant to 35 U.S.C. §111(b).

TECHNICAL FIELD

The present invention relates to a Group III nitride semiconductordevice, exhibiting good crystallinity, which is employed in, forexample, light-emitting diodes, laser diodes, and electronic devices.

BACKGROUND ART

Group III nitride semiconductors have a direct transition bandstructure-and exhibit bandgap energies corresponding to the energy ofvisible to ultraviolet light. By virtue of these characteristics, GroupIII nitride semiconductors are employed at present for producinglight-emitting devices, including blue LEDs, blue-green LEDs,ultraviolet LEDs, and white LEDs (which contain a fluorescent substancein combination with such a nitride semiconductor).

Growing only a nitride-single crystal itself has been considereddifficult, for the following reasons. Nitrogen, which is a constituentof the single crystal, has high dissociation pressure and thereforefails to be retained in the single crystal in, for example, the pullingmethod.

Therefore, a Group III nitride semiconductor is generally produced bymeans of metal organic chemical vapor deposition (MOCVD). In thistechnique, a single-crystal substrate is placed on a heatable jigprovided in a reaction space, and raw material gases are fed onto thesurface of the substrate, to thereby grow, on the substrate, anepitaxial film of nitride semiconductor single crystal. Thesingle-crystal substrate is formed of, for example, sapphire or siliconcarbide (SiC). However, even when a nitride semiconductor single crystalis grown directly on such a single-crystal substrate, large amounts ofcrystal defects, which are attributed to crystal lattice mismatchbetween the crystalline substrate and the single crystal, are generatedin the resultant nitride semiconductor single crystal film; i.e., theepitaxial film fails to exhibit good crystallinity. In view of theforegoing, there have been proposed several methods for growing, betweena substrate and a nitride semiconductor single crystal epitaxial film, alayer having a function for suppressing generation of crystal defects(i.e., a layer corresponding to a buffer layer), so as to attain goodcrystallinity of the epitaxial film.

In one typical method, an organometallic raw material and a nitrogensource are simultaneously fed onto a substrate at a temperature of 400to 600° C., to thereby form a low-temperature buffer layer; thethus-formed buffer layer is subjected to thermal treatment (i.e.,crystallization) at an increased temperature; and a target Group IIInitride semiconductor single crystal is epitaxially grown on theresultant buffer layer (see Japanese Patent Application Laid-Open(kokai) No. 2-229476). Also, there has been proposed a method includinga first step of depositing fine Group III metal particles onto thesurface of a substrate; a second step of nitridizing the fine particlesin an atmosphere containing a nitrogen source; and a third step ofgrowing a target Group III nitride semiconductor single crystal on thethus-nitridized fine particles (see International Publication WO02/17369 Pamphlet).

Such a method can produce a Group III nitride semiconductor singlecrystal exhibiting somewhat good crystallinity. However, with an aim tofurther improve in the performance of a semiconductor device, demandstill exists for a Group III nitride semiconductor crystal exhibitingfurther enhanced crystallinity.

Important factors for evaluating the performance of a semiconductorlight-emitting device are, for example, emission wavelength, emissionintensity and forward voltage under application of rated current, andreliability of the device. A key indicator for determining such areliability is whether or not current flows under application of reversevoltage (not forward voltage) to the device; i.e., the magnitude of thethreshold voltage at which reverse current begins to flow. Suchthreshold voltage is called “reverse withstand voltage.” In recentyears, demand has arisen for a semiconductor light-emitting deviceexhibiting higher reverse withstand voltage and, accordingly, a furtherimprovement in crystallinity is required.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a Group III nitridesemiconductor device exhibiting improved crystallinity and goodperformance. Another object of the present invention is to provide aGroup III nitride semiconductor light-emitting device exhibiting highreverse withstand voltage; i.e., a high threshold voltage at which areverse current begins to flow.

The present invention provides the following.

-   (1) A Group III nitride semiconductor device comprising a substrate,    and a plurality of Group III nitride semiconductor layers provided    on the substrate, wherein a first layer which is in contact with the    substrate is composed of silicon-doped Al_(x)Ga_(1-x)N (0≦x≦1).-   (2) A Group III nitride semiconductor device according to (1) above,    wherein the first layer contains silicon in an amount of 1×10¹⁶ to    1×10¹⁹ atoms/cm³.-   (3) A Group III nitride semiconductor device comprising a substrate,    and a plurality of Group III nitride semiconductor layers provided    on the substrate, wherein a first layer which is in contact with the    substrate is composed of Al_(x)Ga_(1-x)N (0≦x≦1), and the difference    in height between a protrusion and a depression which are present at    the interface between the first layer and a second layer provided    thereon is 10 nm or more and is equal to, or less than, 99% the    thickness of the first layer.-   (4) A Group III nitride semiconductor device according to any one    of (1) through (3) above, wherein the first layer has a structure    formed of aggregated columnar crystal grains.-   (5) A Group III nitride semiconductor device according to (4) above,    wherein each of the columnar crystal grains has a width of 10 to 100    nm.-   (6) A Group III nitride semiconductor device according to any one    of (1) through (5) above, wherein the first layer has a thickness of    20 nm to 200 nm.-   (7) A Group III nitride semiconductor light-emitting device    comprising a substrate; an n-type layer, a light-emitting layer, and    a p-type layer, which are composed of a Group III nitride    semiconductor single crystal and are provided on the substrate in    this order; a negative electrode provided on the n-type layer; and a    positive electrode provided on the p-type layer, wherein there is a    layer composed of silicon-doped Al_(x)Ga_(1-x)N (0≦x≦1) in contact    with the substrate.-   (8) A Group III nitride semiconductor light-emitting device    according to (7) above, wherein the silicon-doped Al_(x)Ga_(1-x)N    (0≦x≦1) layer has a structure formed of aggregated columnar crystal    grains.-   (9) A method for producing a Group III nitride semiconductor device,    which method comprises a first step of depositing, on the surface of    a substrate, a layer containing fine Group III metal particles    containing silicon; a second step of nitridizing the fine particles    in an atmosphere containing a nitrogen source; and a third step of    growing a Group III nitride semiconductor single crystal on the    thus-nitridized fine particles.-   (10) A method for producing a Group III nitride semiconductor device    according to (9) above, which further comprises, between the first    and second steps, an annealing step of heating the fine particles in    an atmosphere containing hydrogen gas and/or nitrogen gas.

According to the present invention, there are obtained a Group IIInitride semiconductor device of improved crystallinity, and a Group IIInitride semiconductor light-emitting device exhibiting high reversewithstand voltage; i.e., a high threshold voltage at which reversecurrent begins to flow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an electron micrograph showing the cross section of the GroupIII nitride semiconductor light-emitting device of Example.

FIG. 2 is a schematic representation of the micrograph shown in FIG. 1.

FIG. 3 is an electron micrograph showing the cross section of the GroupIII nitride semiconductor light-emitting device of Comparative Example.

FIG. 4 is a schematic representation of the micrograph shown in FIG. 3.

FIG. 5 is a schematic representation showing the Group III nitridesemiconductor light-emitting device of the present invention produced inthe Example.

BEST MODE FOR CARRYING OUT THE INVENTION

The Group III nitride semiconductor device provided by the presentinvention includes a substrate, and a plurality of Group III nitridesemiconductor layers provided on the substrate. A characteristic featureof the semiconductor device resides in that a first layer composed ofAl_(x)Ga_(1-x)N (0≦x≦1) which is in contact with the substrate is dopedwith Si. The composition of the first layer, which is formed of acompound containing N, and Al and/or Ga, may be appropriately determinedin accordance with the type of a Group III nitride semiconductor singlecrystal constituting second and subsequent layers which are to be grownon the first layer. The first layer may be formed solely of AlN (i.e., acompound containing no Ga), or solely of GaN (i.e., a compoundcontaining no Al).

Preferably, the first layer has a structure formed of aggregatedcolumnar crystal grains. As used herein, the term “columnar crystalgrain” refers to a crystal grain which has a columnar vertical crosssection and is separated from an adjacent crystal grain by a grainboundary formed therebetween.

FIG. 1 is an electron micrograph showing the cross section of alight-emitting device of Example 1, and FIG. 2 is a schematicrepresentation of the electron micrograph shown in FIG. 1. Referencenumeral 1 denotes a substrate, 2 a first layer, and 3 a second layerwhich is grown on the first layer and is formed of undoped GaN singlecrystal. These figures show that the first layer is formed of aggregatedcolumnar crystal grains. Reference numerals 21, 22, and 23 denotecolumnar crystal grains.

The second layer 3, which is formed of undoped GaN single crystal, isepitaxially grown on the first layer 2, and epitaxial growth of thesecond layer starts at several columnar crystal grains selected fromamong all the crystal grains constituting the first layer. The columnarcrystal grains at which the layer growth starts are selected inaccordance with the shape of the surface of the first layer;specifically, the difference in height between adjacent columnar crystalgrains of the first layer.

As used herein, the expression “the height (h) of a columnar crystalgrain” is defined by the distance between the upper surface A of thesubstrate 1 and the interface B between the first and second layers asmeasured, through observation under an electron microscope, at aposition where the columnar crystal grain, which constitutes the firstlayer, is present (see FIGS. 1 and 2). The absolute value of the height(h) of the columnar crystal grain is calculated on the basis of themagnification of the electron microscope during the course ofobservation. The difference in height between the highest columnarcrystal grain and the lowest columnar crystal grain constituting thefirst layer of a sample, which difference is measured by use of aelectron micrograph (size: 10 cm) captured at an arbitrary portion ofthe first layer at a magnification of 500,000, is defined as the maximumdifference in height between protrusions and depressions present at thesurface of the first layer of the sample (i.e., the maximum differencein height between protrusions and depressions present at the interfacebetween the first and second layers of the sample).

The conditions of the surface of the first layer may be an importantfactor for determining the crystallinity of a Group III nitridesemiconductor single crystal layer to be grown on the first layer,although the crystallinity of the single crystal layer may be affectedby the growth conditions therefor.

The present inventor has found that the maximum difference in heightbetween protrusions and depressions present at the surface of the firstlayer is preferably 10 nm or more and is equal to, or less than, 99% thethickness of the first layer, more preferably 10 nm or more and equal toor less than 90% the thickness of the first layer. When the maximumheight difference falls within the above range, a Group III nitridesemiconductor single crystal of good crystallinity is epitaxially grownon the first layer. When the maximum height difference is smaller than10 nm, single crystal growth starts at a greater number of selectedcrystal grains, which is not desirable from the viewpoint of control ofthe crystal growth. In contrast, when the maximum height difference isexcessively large, a Group III nitride semiconductor single crystalhaving a mirror surface fails to be obtained. Thus, the maximum heightdifference is preferably 60 nm or less, more preferably 40 nm or less.

When the width of the columnar crystal grains (see FIG. 2) is small,epitaxial growth of a Group III nitride semiconductor single crystalstarts at numerous columnar crystal grains, possibly leading to crystalgrowth in different orientations; i.e., random crystal growth.Therefore, the columnar crystal grain width is preferably 10 nm or more.However, when the columnar crystal grain width is excessively large,single crystal growth fails to start at the columnar crystal grains.Therefore, the columnar crystal grain width is preferably 100 nm orless. The columnar crystal grain width is more preferably 20 to 60 nm,particularly preferably 20 to 40 nm.

The present inventor has found that when the first layer is doped withsilicon, the maximum difference in height between protrusions anddepressions present at the surface of the first layer becomes 10 nm ormore. In the case where the first layer is not doped with silicon, themaximum difference in height between protrusions and depressions presentat the surface of the first layer becomes smaller than 10 nm, andtherefore, the crystallinity of a Group III nitride semiconductor to begrown on the first layer is deteriorated as compared with the case wherethe first layer is doped with silicon. The doping amount of silicon ispreferably 1×10¹⁶ to 1×10¹⁹ atoms/cm³, more preferably 1×10¹⁶ to 1×10¹⁸atoms/cm³, particularly preferably 1×10¹⁶ to 5×10¹⁷ atoms/cm³. When thesilicon doping amount is less than 1×10¹⁶ atoms/cm³, the considerableeffects of silicon doping are not obtained, whereas when the silicondoping amount is more than 1×10¹⁹ atoms/cm³, the columnar crystal grainsfail to be maintained, which is not preferred.

The thickness of the first layer is an important parameter. Within thecontext of the present invention, the thickness of the thickest portionof the first layer is defined as the thickness of the first layer,although protrusions and depressions are present at the interfacebetween the first and second layers; i.e., the thickness of the firstlayer differs from portion to portion. The thickness of the first layeris preferably 20 nm or more, more preferably 40 nm or more. When thethickness is smaller than 20 nm, difficulty is encountered in securingthe difference in height between protrusions and depressions present atthe surface of the first layer. No particular limitations are imposed onthe maximum value of the thickness of the first layer. However, evenwhen the thickness of the first layer is increased to 200 nm or more,epitaxial growth of a Group III nitride semiconductor crystal on thefirst layer is not considerably affected by the layer thickness. Inaddition, when the thickness of the first layer is to be increased to alevel more than necessary, a long period of time is required for growththereof, which is not desirable. The thickness of the first layer ispreferably regulated to 100 nm or less.

The composition and configuration of the second and subsequent Group IIInitride semiconductor layers, which are to be grown on the first layer,are appropriately selected in accordance with the intended use of theresultant semiconductor device. For example, in the case where thesemiconductor device is a light-emitting device, an n-type layer, alight-emitting layer, and a p-type layer are formed on the first layerin this order, and a negative electrode and a positive electrode areformed on the n-type layer and the p-type layer, respectively.

As for the second and subsequent Group III nitride semiconductor layers,conventionally known gallium nitride-based compound semiconductorsrepresented by formula: Al_(x)Ga_(y)In_(1-x-y)N (0≦x≦1, 0≦y≦1, x+y≦1)may be employed.

No particular limitations are imposed on the method for forming theseGroup III nitride semiconductor layers, and there may be employed anyknown method for growing a Group III nitride semiconductor, such asMOCVD (metal organic chemical vapor deposition), HVPE (hydride vaporphase epitaxy), or MBE (molecular beam epitaxy). From the viewpoints oflayer thickness controllability and mass productivity, MOCVD ispreferably employed.

In the case where the Group III nitride semiconductor layers are grownby means of MOCVD, hydrogen (H₂) or nitrogen (N₂) is employed as acarrier gas. Such a carrier gas is caused to pass through a purificationapparatus for removing, for example, oxygen or moisture, and then fedinto a reactor. Through valve switching, and under the control of a massflow controller, the composition of the carrier gas can be rapidlychanged from 100% nitrogen to 100% hydrogen. The carrier gas to be fedinto the reactor may be a gas mixture of nitrogen and hydrogen. In thiscase, the compositional proportions of nitrogen and hydrogen can bearbitrarily determined.

Examples of the Group III metal source to be employed includeorganometallic compounds such as trimethylgallium (TMGa),triethylgallium (TEGa), trimethylaluminum (TMAl), triethylaluminum(TEAl), trimethylindium (TMIn), and triethylindium (TEIn), which areappropriately determined in accordance with the composition of therespective semiconductor layers. Such an organometallic compound issealed in a cylindrical metallic container, and the container is placedin a thermostatic chamber whose temperature can be maintained at apredetermined level. A Group III metal source is fed into the reactor asdescribed below.

When a carrier gas is fed into the metallic container through a metallictube provided in the container, the Group III metal source is subjectedto bubbling in the container by means of the thus-fed carrier gas, andthe vapor of the metal source is contained in the carrier gas. Thecarrier gas containing the organometallic compound (Group III metalsource) is fed into the reactor through an automaticallyopenable/closable valve under control of a mass flow controller. As usedherein, “mass flow controller” refers to an apparatus enabling to feed,into the reactor, a necessary amount of the metal source to be employed.In the present invention, the mass flow controller can be used forfeeding of the Group III metal source and the carrier gas employed.

The nitrogen source to be employed is preferably ammonia (NH₃),hydrazine (N₂H₄), or a similar material. In the case where the nitrogensource is ammonia, ammonia gas obtained through vaporization of liquidammonia contained in a cylinder is employed. The feed amount of theammonia gas is regulated to a predetermined level through flow ratecontrol by means of a mass flow controller, and the ammonia gas is fedinto the reactor through a metallic tube capable of feeding the ammoniagas together with a carrier gas, and through an automaticallyopenable/closable valve.

The n-type dopant to be employed may be, for example, monosilane (SiH₄),disilane (Si₂H₆), or germane (GeH₄), whereas the p-type dopant to beemployed may be, for example, bis(cyclopentadienyl)magnesium (Cp₂Mg) orbis (ethylcyclopentadienyl)magnesium (EtCp₂Mg). When monosilane isemployed, it is fed into the reactor in a manner similar to the case ofammonia, whereas when Cp₂Mg is employed, it is fed into the reactor in amanner similar to the case of the organometallic source.

The reactor is heated by means of heating a carbon-made susceptor onwhich a substrate is to be placed, which susceptor generates heatthrough induction current generated therein by applying electricity toan induction heating RF coil provided below the susceptor. Thetemperature of the susceptor is measured by means of a pyrometerprovided in the vicinity of the bottom surface of the susceptor, and thethus-measured temperature data are converted into control signals. Thecontrol signals are sent to a feedback mechanism for supplying electricpower to the RF coil, whereby the susceptor temperature is controlled toa predetermined level.

Preferably, the first layer is formed by means of the method including afirst step of depositing silicon-containing fine Group III metalparticles onto the surface of a substrate, and a second step ofnitridizing the fine particles in an atmosphere containing a nitrogensource. More preferably, an annealing step for heating the fineparticles in a carrier gas atmosphere is carried out between the firstand second steps.

In the first step, preferably, the ratio by mole (concentration) ofammonia gas (nitrogen source) to a Group III metal source (i.e.,ammonia/Group III metal source ratio) is regulated to 1,000 or less.When this ratio is higher than 1,000, reaction between ammonia and themetal source proceeds considerably before these materials reach thesubstrate, and fine Group III metal particles may fail to be formed onthe substrate. The ammonia/Group III metal source ratio may be zero;i.e., feeding of ammonia gas may be omitted, as the fine Group IIIparticles are sufficiently nitridized in the second step, or nitrogenobtained through decomposition of a nitride (e.g., polycrystalline GaN),which is deposited onto the inner side walls and ceiling of the reactor,and onto the surface of the susceptor on which a substrate is to beplaced, partially contributes to nitridation of the fine Group III metalparticles deposited onto the first layer. In order to attain consistentquality of the first layer, preferably, the amount of a nitride, whichis deposited onto the inner side walls and ceiling of the reactor, andonto the surface of the susceptor on which a substrate is to be placed,is maintained at a constant level. Therefore, preferably, the final stepof the process for forming Group III nitride semiconductor layers isalways completed under the same Group III nitride semiconductor growthconditions.

In some cases, only hydrogen or a gas mixture of hydrogen and ammoniagas is caused to flow through the reactor during heating. This process,which is called “baking,” is carried out for the purpose of removingexcess deposition products present in the reactor. After completion ofbaking, nitride deposits are removed from the reactor. Therefore, beforeformation of the first layer, preferably, a nitride layer formed ofpolycrystalline GaN is deposited in advance through growth on the innerside walls and ceiling of the reactor, and on the surface of thesusceptor on which a substrate is to be placed.

The first step is preferably carried out at a temperature of 950 to1,250° C. When the first step is carried out at a temperature higherthan 1,250° C., fine Group III metal particles tend to disperseimmediately after being deposited onto the surface of a substrate due tohigh temperature, and difficulty is encountered in forming the firstlayer. In contrast, when the first step is carried out at a temperaturelower than 950° C., fine Group III metal particles are excessivelydeposited onto the surface of a substrate, which is not preferred. Thefirst step is more preferably carried out at 1,000 to 1,200° C., morepreferably at 1,000° C. to 1,170° C., particularly preferably at 1,040to 1,120° C.

The second step is preferably carried out at a temperature of 1,050 to1,250° C. When the second step is carried out at a temperature higherthan 1,250° C., the fine Group III metal particles which have beendeposited onto a substrate in the first step tend to disperse from thesubstrate, and control of the particles becomes difficult. In contrast,when the second step is carried out at a temperature lower than 1,050°C., nitridation of the fine Group III metal particles proceeds rapidly,and control of the nitridation becomes difficult. Needless to say, bothcases are not preferred. The second step is more preferably carried outat 1,100 to 1,200° C., particularly preferably at 1,100 to 1,150° C.

In the case where the annealing step is carried out between the firstand second steps, the annealing step is carried out at the temperatureat the time when the first step is completed. This annealing steppromotes distribution of the fine Group III metal particles, therebyforming fine Group III metal particles of more desirable shape.

The second step is followed by a third step of growing a Group IIInitride semiconductor single crystal layer (e.g., an undoped GaN layer).

No particular limitations are imposed on the material of a substrate,and the substrate may be formed of any known material. Examples of theknown material include oxide single crystals such as sapphire singlecrystal (Al₂O₃; A-plane, C-plane, M-plane, or R-plane), spinel singlecrystal (MgAl₂O₄), ZnO single crystal, LiAlO₂ single crystal, LiGaO₂single crystal, and MgO single crystal; Si single crystal; SiC singlecrystal; GaAs single crystal; AlN single crystal; GaN single crystal;and boride single crystals such as ZrB₂ single crystal. Preferably, asapphire substrate or an SiC substrate is employed. No particularlimitations are imposed on the crystal orientation of the substrate.However, when a sapphire substrate is employed, the crystal orientationis preferably C-plane ((0001) plane). Preferably, an axis perpendicularto the surface of the sapphire substrate is inclined by a specific anglewith respect to the <0001> plane of the substrate.

EXAMPLE

The present invention will next be described in detail by way of anExample, which should not be construed as limiting the invention.

[Example]

FIG. 5 is a schematic representation showing the Group III nitridesemiconductor light-emitting device of the present invention produced inthe present Example. Reference numeral 1 denotes a substrate composed ofsapphire. Semiconductor layers were grown along the crystal planeorientation of the substrate. A first layer 2 composed ofAl_(0.08)Ga_(0.92)N doped with Si (5×10¹⁸ atoms/cm³) was formed on thesubstrate. A second layer 3 composed of an undoped GaN single crystallayer, and an n-type GaN single crystal layer 4 doped with Si (1×10¹⁹atoms/cm³) were successively formed on the first layer 2. Alight-emitting layer 6 was formed, via a Ga_(0.98)In_(0.02)N layer 5, onthe Si-doped GaN single crystal layer 4. The Ga_(0.98)In_(0.02)N layer 5is provided between the Si-doped GaN layer 4 and the light-emittinglayer 6 for preventing propagation of crystal defects from the Si-dopedGaN layer 4.

The light-emitting layer 6 has a structure including a plurality ofstacked layer units, each including a barrier layer composed of anundoped GaN layer and a well layer composed of a Ga_(0.92)In_(0.08)Nlayer (i.e., a layer composed of GaN and InN). In the present Example,five layer units were laminated. The GaN barrier layer has a thicknessof 75 nm, and the GaInN well layer has a thickness of 25 nm (totalthickness: 100 nm). A barrier layer having a thickness of 75 nm waslaminated on the outermost layer (well layer) of the layer units, tothereby form the light-emitting layer.

A p-type Al_(0.1)Ga_(0.9)N layer 7 doped with Mg (3×10¹⁹ atoms/cm³) anda p-type GaN layer 8 doped with Mg (6×10¹⁹ atoms/cm³) were formed on thelight-emitting layer. Reference numeral 9 denotes a positive electrode,which has a four-layer structure formed of successively laminated Au,Ti, Al, and Au layers. Reference numeral 10 denotes a negativeelectrode, which has a four-layer structure formed of successivelylaminated Ni, Al, Ti, and Au layers.

The Group III nitride semiconductor light-emitting device was producedthrough the following procedure. Firstly, a C-plane sapphire substratewas placed on a carbon-made susceptor provided in the center of areactor of an MOCVD production apparatus. While the substrate was placedon the susceptor, nitrogen gas was caused to flow through the reactor.After the substrate was placed on the susceptor, the lid of the reactorwas closed, and hydrogen gas was fed into the reactor for five minutes,to thereby completely replace the nitrogen gas contained in the reactorwith hydrogen gas. Thereafter, while hydrogen gas was caused to flowthrough the reactor, the carbon-made susceptor was heated by means of aninduction heating RF coil provided below the susceptor. Specifically,the temperature of the susceptor was elevated to 600° C. over sixminutes, and subsequently, the temperature elevation operation wasstopped when the susceptor temperature reached 600° C., and thesusceptor temperature was maintained at 600° C. for 10 minutes.

(Formation of first layer 2)

After 10 minutes elapsed, the susceptor temperature was elevated from600° C. to 1,040° C. over six minutes. When the temperature reached1,040° C., TMAl, TMGa, and SiH₄ were fed into the reactor, to therebyinitiate a first step of forming, on the substrate, a layer throughdeposition of Si-doped fine particles of Group III metals (Al and Ga).While the above raw materials were fed into the reactor, the susceptortemperature was elevated from 1,040° C. to 1,120° C. over five minutes.When the susceptor temperature reached 1,120° C., the temperatureelevation was stopped, and the susceptor temperature was maintained at1,120° C. for five minutes. The time for the first step is 10 minutes(i.e., total of the time for the temperature elevation from 1,040° C. to1,120° C. (five minutes) and the time for maintaining the susceptortemperature at 1,120° C. (five minutes)). During the first step, thefeed amounts of SiH₄, TMGa, and TMAl gases were regulated such that theratio by mole (concentration) of Si to (Ga+Al) became 10⁻⁴, and thefirst layer formed on the substrate was doped with a predeterminedamount of Si (the feed amounts of these raw material gases had beenpredetermined on the basis of the results of SIMS analysis, whichresults indicate that when the ratio by mole of Si to (Ga+Al) satisfiesthe above value, the Si content of the first layer becomes 5×10¹⁸atoms/cm³).

Ten minutes after the start of the first step, feeding of TMAl, TMGa,and SiH₄ was stopped. Subsequently, while the susceptor temperature wasmaintained at 1,120° C., only hydrogen gas, serving as a carrier gas,was fed into the reactor, and an annealing treatment was initiated. Thisannealing treatment is performed for the purpose of promotingdistribution of the fine Group III metal particles deposited onto thesubstrate, thereby forming fine Group III metal particles of moredesirable shape. The annealing treatment was performed for threeminutes.

After the above annealing treatment had been performed for threeminutes, ammonia gas was fed into the reactor, and a second step; i.e.,an annealing treatment under a stream of ammonia gas, was initiated.This annealing treatment is performed for the purpose of completelynitridizing non-nitridized, fine Group III metal particles. Theannealing treatment was performed for eight minutes. After eight minuteselapsed, the susceptor temperature was lowered from 1,120° C. to 1,040°C. over two minutes. Meanwhile, the feed amount of ammonia gas waschanged from 15 L/minute to 12 L/minute.

(Formation of undoped GaN layer 3 and Si-doped GaN layer 4)

Thereafter, the susceptor temperature was confirmed to be 1,040° C., andTMGa was fed into the reactor, to thereby initiate growth of the undopedGaN layer on the first layer. The feed amount of TMGa was regulated suchthat the rate of growth of the GaN layer became 2.0 μm/hour. Aftergrowth of the undoped GaN layer was performed for one hour, a valve forfeeding SiH₄ was opened, and growth of the Si-doped GaN layer wasinitiated. The feed amount of SiH₄ was regulated such that the ratio bymole of Si to Ga became 10⁻⁴. After growth of the Si-doped GaN layer wasperformed for one hour, the valves for feeding TMGa and SiH₄ wereclosed, and feeding of these materials was stopped, to thereby stopgrowth of the Si-doped GaN layer. Meanwhile, feeding of ammonia wascontinued. Thereafter, the hydrogen carrier gas was completely replacedby a nitrogen carrier gas, and the susceptor temperature was loweredfrom 1,040° C. to 755° C. over seven minutes. During the course of thistemperature lowering, the feed amounts of TMIn and TEGa, which areemployed for growth of the GaInN layer 5, were regulated. Meanwhile, thefeed amount of SiH₄ was regulated such that the ratio by mole of Si toGa became 10⁻⁵.

The undoped GaN layer and the Si-doped GaN layer were found to have athickness of 2.0 μm (i.e., total of the thicknesses of these layers wasfound to be 4.0 μm). The Si content of the Si-doped GaN layer was foundto be 1×10¹⁹ atoms/cm³.

(Formation of Ga_(0.98)In_(0.02)N layer 5)

After seven minutes had elapsed, the susceptor temperature was confirmedto be maintained at 755° C. Subsequently, valves for feeding TEGa, TMIn,and SiH₄ were opened, and these materials were fed into the reactor, tothereby initiate growth of the Ga_(0.98)In_(0.02)N layer under nitrogencarrier gas flow. After 50 minutes had elapsed, the valves for feedingTEGa, TMIn, and SiH₄ were closed, and growth of the Ga_(0.98)In_(0.02)Nlayer was stopped. The resultant Ga_(0.98)In_(0.02)N layer was found tohave a thickness of 25 nm, and to be doped with Si in an amount of3×10¹⁸ atoms/cm³.

(Formation of light-emitting layer 6)

Subsequently, there was repeated the following procedure: a valve forfeeding TEGa is opened, and a barrier layer composed of an undoped GaNlayer is grown; a valve for feeding TMIn is opened when the thickness ofthe barrier layer becomes 75 nm, to thereby feed TMIn into the reactorand grow a well layer composed of a Ga_(0.92)In_(0.08)N layer; and thevalve for feeding TMIn is closed when the thickness of theGa_(0.92)In_(0.08)N well layer becomes 25 nm. Specifically, the abovecombined procedure was carried out five times, to thereby laminate fivebarrier layers and five well layers alternately. A barrier layer wasgrown on the outermost well layer of the thus-formed laminate.

(Formation of Mg-doped Al_(0.1)Ga_(0.9)N layer 7)

Through valve switching, the nitrogen carrier was replaced with ahydrogen carrier gas. The susceptor temperature was elevated from 755°C. to 1,020° C. over four minutes. During the course of this temperatureelevation, the feed amounts of TMGa, TMAl, and Cp₂Mg were regulatedthrough control of mass flow controllers for these materials. In thiscase, the feed amounts of TMGa and TMAl gases were regulated such thatthe ratio by mole of Al to Ga became 0.2. Meanwhile, the feed amount ofCp₂Mg was regulated such that the ratio by mole of Mg to Ga became 0.25.After the susceptor temperature was confirmed to be maintained at 1,020°C., valves for feeding TMGa, TMAl, and Cp₂Mg were opened, and growth ofthe Mg-doped AlGaN layer was initiated. After the layer growth wasperformed for one minute, the valves for feeding TMGa, TMAl, and Cp₂Mgwere closed, whereby growth of the layer was stopped. The resultantMg-doped AlGaN layer was found to have a thickness of 10 nm and an Mgcontent of 3×10¹⁹ atoms/cm³.

(Formation of Mg-doped GaN layer 8)

Subsequently, the feed amounts of TMGa and Cp₂Mg were regulated throughcontrol of mass flow controllers for these materials. In this case, thefeed amounts of TMGa and Cp₂Mg gases were regulated such that the ratioof Mg to Ga became 0.5. Regulation of the feed amounts of thesematerials was continued for two minutes until the Mg/Ga ratio becameconstant. Thereafter, valves for feeding TMGa and Cp₂Mg were opened, tothereby grow the Mg-doped GaN layer having a thickness of 0.1 μm and anMg content of 6×10¹⁹ atoms/cm³.

After completion of growth of the Mg-doped GaN layer, application ofelectricity to the induction heating RF coil was stopped, to therebylower the susceptor temperature from 1,020° C. to room temperature over20 minutes. After initiation of the temperature lowering, the carriergas composition was changed to 100% nitrogen gas, and the feed amount ofammonia was reduced to 1/100 of that during the course of growth of theMg-doped GaN layer. When the susceptor temperature was lowered to 300°C., feeding of ammonia was stopped; i.e., only nitrogen gas was causedto flow through the reactor. When the susceptor temperature was loweredto room temperature, the resultant wafer was removed from the reactorinto the air. The entire surface of the wafer was found to have a mirrorsurface. The Mg-doped GaN layer exhibited p-type conductivity withoutannealing treatment for activation. Thus, the wafer was produced byforming the Group III nitride semiconductor epitaxial layers on thesapphire substrate.

The structure of the first layer of the wafer was observed under anelectron microscope at a magnification of 500,000. FIG. 1 is amicrograph showing the cross section of the first layer, and FIG. 2 is aschematic representation of the micrograph shown in FIG. 1. Observationof the first layer revealed that the layer was a polycrystalline layerformed of aggregated of columnar crystal grains having a width fallingwithin a range of 30 to 50 nm, and that the maximum difference in heightbetween protrusions and depressions present at the surface of the firstlayer (i.e., the interface between the first layer and the undoped GaNlayer) was about 30 nm.

By use of the wafer produced through the above-described procedure, theGroup III nitride semiconductor light-emitting device was producedthrough the following procedure. Firstly, by means of lithography, atechnique which is well known in the art, a translucent gold electrodelayer was formed on the surface of the Mg-doped GaN layer, and atitanium layer, an aluminum layer, and a gold layer were successivelylaminated on the electrode layer, to thereby form an electrode bondingpad. They serve as a positive electrode. Subsequently, the wafer wassubjected to dry etching until a portion of the Si-doped GaN layer wasexposed to the outside. On the thus-exposed portion, a nickel layer, analuminum layer, a titanium layer, and a gold layer were successivelylaminated, to thereby form a negative electrode.

The back surface of the sapphire substrate of the wafer including theabove-formed positive and negative electrodes was subjected to grindinguntil the thickness of the wafer became a predetermined level and,subsequently, the thus-ground back surface was subjected to polishingsuch that the back surface became a mirror surface. The resultant waferwas cut into chips, each having a size of 350 μm×350 μm and includingone positive electrode and one negative electrode. The resultant chipwas fixated on a stem such that the positive and negative electrodesfaced upward, and the chip was connected to a lead frame by means of agold wire, to thereby produce the light-emitting device.

When a forward current of 20 mA was caused to flow between the positiveand negative electrodes of the light-emitting device, the deviceexhibited a forward voltage of 3.2 V, emitted light having a wavelengthof 460 nm, and exhibited an emission output of 6 cd. When a reversecurrent of 10 μA was caused to flow between the positive and negativeelectrodes of the light-emitting device, the reverse withstand voltagewas found to be 16 V.

COMPARATIVE EXAMPLE

The procedure of the Example was repeated, except that the first step offorming the first layer was performed without feeding of SiH₄, tothereby produce a light-emitting device.

In a manner similar to that of the Example, the first layer of thethus-produced light-emitting device was observed under an electronmicroscope. FIG. 3 is a micrograph showing the cross section of thefirst layer, and FIG. 4 is a schematic representation of the micrographshown in FIG. 3. Observation of the first layer revealed that the layerwas a polycrystalline layer formed of aggregated columnar crystal grainshaving a width falling within a range of 30 to 50 nm, and that themaximum difference in height between protrusions and depressions presentat the surface of the first layer (i.e., the interface between the firstlayer and the undoped GaN layer) was as small as 10 nm or less.

When a forward current of 20 mA was caused to flow between the positiveand negative electrodes of the light-emitting device, the deviceexhibited a forward voltage of 3.2 V, emitted light having a wavelengthof 460 nm, and exhibited an emission output of 6 cd. When a reversecurrent of 10 μA was caused to flow between the positive and negativeelectrodes of the light-emitting device, the reverse withstand voltagewas found to be 10 V.

INDUSTRIAL APPLICABILITY

When the Group III nitride semiconductor device of the present inventionis employed in a light-emitting device (e.g., a light-emitting diode ora laser diode) or an electronic device, the resultant device can attainvery high efficiency. Therefore, the Group III nitride semiconductordevice has very high industrial utility value.

1. A Group III nitride semiconductor device comprising: a sapphiresubstrate; and a plurality of Group III nitride semiconductor layersprovided on the substrate, wherein a first layer which is in contactwith the substrate is composed of silicon-doped Al_(x)Ga_(1-x)N (0<x≦1)and has a structure formed of aggregated columnar crystal grains havinga width of 10 to 50 nm.
 2. A Group III nitride semiconductor deviceaccording to claim 1, wherein the first layer contains silicon in anamount of 1×10¹⁶ to 1×10¹⁹ atoms/cm³.
 3. A Group III nitridesemiconductor device comprising: a substrate; and a plurality of GroupIII nitride semiconductor layers provided on the substrate, including afirst layer which is in contact with the substrate and a second layergrown on said first layer, wherein said first layer which is in contactwith the substrate is composed of Al_(x)Ga_(1-x)N (0≦x≦1), and thedifference in height between a protrusion and a depression which arepresent at the interface between the first layer and the second layer is10 nm or more and is equal to, or less than, 99% the thickness of thefirst layer.
 4. A Group III nitride semiconductor device according toclaim 1, wherein the first layer has a thickness of 20 nm to 200 nm. 5.A Group III nitride semiconductor light-emitting device comprising: asubstrate; an n-type layer, a light-emitting layer, and a p-type layer,which are composed of a Group III nitride semiconductor single crystaland are provided on the substrate in this order; a negative electrodeprovided on the n-type layer; and a positive electrode provided on thep-type layer, wherein there is a layer composed of silicon-dopedAl_(x)Ga_(1-x)N (0<x≦1) in contact with the substrate and the layer incontact with the substrate has a structure formed of aggregated columnarcrystal grains having a width of 10 to 50 nm.
 6. A method for producinga Group III nitride semiconductor device comprising a sapphiresubstrate, and a plurality of Group III nitride semiconductor layersprovided on the substrate, wherein a first layer which is in contactwith the substrate is composed of silicon-doped Al_(x)Ga_(1-x)N (0<x<1)and has a structure formed of aggregated columnar crystal grains havinga width of 10 to 50 nm, which method comprises a first step ofdepositing, on the surface of the substrate, a layer containing fineGroup III metal particles containing silicon; a second step ofnitridizing the fine particles in an atmosphere containing a nitrogensource; and a third step of growing a Group III nitride semiconductorsingle crystal on the thus-nitridized fine particles.
 7. A method forproducing a Group III nitride semiconductor device according to claim 6,which further comprises, between the first and second steps, anannealing step of heating the fine particles in an atmosphere containinghydrogen gas and/or nitrogen gas.
 8. A Group III nitride semiconductordevice according to claim 1, wherein the first layer is composed ofsilicon-doped Al_(x)Ga_(1-x)N (0<x<1).
 9. A Group III nitridesemiconductor light-emitting device according to claim 5, wherein thelayer in contact with the substrate is composed of silicon-dopedAl_(x)Ga_(1-x)N (0<x<1).
 10. A Group III nitride semiconductor deviceaccording to claim 1, wherein the columnar crystal grains have a widthof 20 to 40 nm.
 11. A Group III nitride semiconductor device accordingto claim 1, wherein the first layer is a continuous layer formed by acontinuous formation of the columnar crystal grains.
 12. A Group IIInitride semiconductor device according to claim 3, wherein the thicknessof the first layer is a thickness of a thickest portion of the firstlayer which has a maximum height of the first layer.
 13. A Group IIInitride semiconductor device according to claim 3, wherein thedifference in height between the protrusion and the depression is amaximum height difference between a maximum protrusion and a maximumdepression of an upper surface of the first layer.
 14. A Group IIInitride semiconductor device according to claim 13, wherein the maximumprotrusion and the maximum depression of the upper surface of the firstlayer is a thickest portion and a least thickest portion of the firstlayer, respectively.
 15. A Group III nitride semiconductor deviceaccording to claim 14, wherein the thickness of the first layer is thethickest portion of the first layer having a maximum height.
 16. A GroupIII nitride semiconductor device according to claim 14, wherein thedifference in height between the protrusion and the depression is 40 nmor less.
 17. A Group III nitride semiconductor device according to claim14, wherein the columnar crystal grains have a width of 20 to 40 nm. 18.A Group III nitride semiconductor device according to claim 3, whereinthe difference in height between the protrusion and the depression is 10nm or more and is equal to, or less than, 90% the thickness of the firstlayer.
 19. A Group III nitride semiconductor device according to claim3, wherein the difference in height between the protrusion and thedepression is 60 nm or less.
 20. A Group III nitride semiconductordevice according to claim 3, wherein the difference in height betweenthe protrusion and the depression is 40 nm or less.